The present invention relates to a reproducing device for an optical recording medium arranged so as to improve reproducing resolution by controlling the size of an opening section for reading out a record mark by means of heat generated by the irradiation of a light beam, using a so-called magnetic super resolution medium, and a control method thereof. More particularly, the present invention relates to an optical reproducing device such as an optical disk device which can optimally control the irradiation strength of a light beam during reproduction, and a control method thereof.
In magneto-optical devices, for a magneto-optical disk of the magnetic super resolution type provided with a recording layer and an in-plane magnetized reproducing layer, a method has been proposed in which a light beam is irradiated to the magneto-optical disk from the reproducing layer side so as to reproduce a record mark smaller than a spot diameter of the light beam. In the foregoing method, a portion of the reproducing layer within an area irradiated by the light beam is heated by the light beam, and the temperature thereof rises above a predetermined level (hereinafter, the portion is referred to as an aperture). Then, to the foregoing portion, the magnetic property of a corresponding portion of the recording layer is copied, and the magnetic property of the foregoing portion of the reproducing layer changes from in-plane magnetization to perpendicular magnetization. In this way, a record mark smaller than the spot diameter of the light beam can be reproduced.
However, in the foregoing method, although the light beam is generated by a constant driving current, there are some cases where an optimal reproducing power of the light beam might vary depending on the changes in the ambient temperature during reproduction, etc. If the value of the reproducing power deviates from the value of the optimal reproducing power, reading errors are more likely to occur. Specifically, if the reproducing power becomes much stronger than the optimal level, the aperture formed becomes too large. Consequently, the output of reproduction signals from tracks adjacent to the track being reproduced is increased, and the proportion of noise signals included in the reproduced data is increased, resulting in the increase in the probability of reading errors. If the reproducing power is much weaker than the optimal level, the aperture formed becomes smaller than the record mark, and the output of the reproduction signals from the track to be read is also reduced, also resulting in the increase in the probability of reading errors.
To cope with the foregoing problem, Japanese Unexamined Patent Publication No. 10-289500/1998 (Tokukaihei 10-289500, published on Oct. 27, 1998) discloses a technique in which two types of patterns for reproducing power control having different mark lengths (short and long) are reproduced, and the reproducing power is controlled in such a manner that a ratio between amplitudes of reproduction signals (hereinafter referred to as a reproduction signal amplitude ratio) obtained from these two patterns gets close to a predetermined optimal value, permitting the reproducing power to be kept at an optimal value and the probability of reading errors to be reduced. Here, the optimal value of the reproduction signal amplitude ratio is obtained by a test read. That is, during the test read, a test pattern is reproduced with the value of the power of a reproducing light beam changed sequentially so as to measure an error rate, and the power of the reproducing light beam having the lowest error rate when reproducing data is obtained as an optimal reproducing power [see FIG. 6(c)]. Then, a ratio Vs/Vl between a reproduction signal amplitude value Vs of a short mark pattern and a reproduction signal amplitude value Vl of a long mark pattern [see FIG. 6(a)] when the optimal reproducing power is provided is determined as an optimal amplitude ratio [see FIG. 6(b)].
In the foregoing conventional technique, the power of the reproducing light beam having the lowest error rate obtained by the test read is determined as the optimal reproducing power. Meanwhile, when controlling the reproducing power, reproducing power control errors are caused by various reasons, including errors of the detected reproduction signal amplitude ratio due to noise, etc.; conversion errors due to the differences in conditions such as tilt, temperature, and defocus, between when the optimal amplitude ratio is obtained and when the reproducing power is actually controlled; and errors in circuits such as a laser driver. That is, there is a possibility that the reproducing power actually generated as a result of the control might deviate from the optimal reproducing power to some extent. Since the direction of the foregoing deviation (whether the reproducing power becomes greater or smaller than the optimal reproducing power) is not fixed, the reproducing power can deviate to both directions.
When the foregoing deviation is caused, as shown in FIG. 6(c), the increase rate of the error rate differs according to whether the reproducing power is greater than the optimal reproducing power or smaller than the optimal reproducing power. This is because the reasons for error increase are different when the reproducing power is greater than the optimal reproducing power and when it is smaller than the optimal reproducing power. The difference in the increase rate of the error rate with respect to the direction of the deviation from the optimal reproducing power is dependent on individual characteristics of a disk and an optical pick up. For example, as shown in FIG. 6(c), in the case where the error rate increases more steeply when the reproducing power is greater than the optimal reproducing power, there is a possibility that the error rate might increase extremely if the reproducing power control error occurs so as to increase the reproducing power (make the reproducing power greater than the optimal reproducing power). That is, since the increase rate of the error rate differs substantially depending on the direction the reproducing power control error occurs, there has been a high possibility that an uncorrectable error might be caused when the reproducing power control error occurs in the direction where the error rate increases extremely.
It is therefore the object of the present invention to provide a reproducing device for an optical recording medium, arranged so as to improve reproducing resolution by controlling the size of an opening section for reading out a record mark by means of heat generated by the irradiation of a light beam, using a so-called magnetic super resolution medium, and more particularly, to provide an optical reproducing device such as an optical disk device which can optimally control the irradiation strength of a light beam during reproduction.
To achieve the foregoing object, an optical reproducing device in accordance with the present invention is structured so as to include:
reproducing means for changing power of a reproducing light beam during test read, and for reproducing test data stored on an optical recording medium for each level of the power;
error detection means for detecting an error rate of the test data that have been reproduced;
error correction means for correcting an error caused during reproduction; and
optimal power determining means for determining an optimal reproducing power based on a condition in which the power of the reproducing light beam in which the error rate that has been detected is not more than an error rate corresponding to a maximum burst error which can be corrected by the error correction means.
According to the foregoing invention, a reproducing light beam is irradiated to an optical recording medium, and based on a reflected light from the optical recording medium, information is reproduced with an error, if any, corrected by the error correction means.
During the test read, the power of the reproducing light beam is changed, the test data stored in the optical recording medium is reproduced by the reproducing means for each level of the power, and the reproducing light beam having the optimal power is determined based on the reproduction result. Here, there are some cases where a control error is caused by the change in the ambient temperature during reproduction, etc., which does not allow the reproducing light beam having the optimal power to be irradiated to the optical recording medium. As a result, the error rate increases and error correction cannot be performed, increasing the probability of reproduction errors.
Hence, according to the foregoing invention, when the optimal power for the reproducing light beam is determined by the optimal power determining means, the error rate detected by the error detection means is determined based on the power of the reproducing light beam in such a manner that the error rate that has been detected is not more than the error rate corresponding to the maximum burst error which can be corrected by the error correction means. With this structure, even if an error is caused in the power control of the reproducing light beam and the error rate increases, the error rate is not more than the error rate corresponding to the maximum burst error correctable by the error correction means, therefore the error caused can be surely corrected by the error correction means.
That is, even if the determined optimal power and an actually emitted power do not match due to a control error and thus the error rate increases, the optimal power for the reproducing light beam is determined so as to prevent the condition that the error rate increases too much and the error cannot be corrected by the error correction means. With this structure, the error correction means can surely correct the error caused, providing a highly reliable optical reproducing device.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.